DESCRIPTION

The Time::HiRes
module implements a Perl interface to the
usleep
, nanosleep
, ualarm
, gettimeofday
, and
setitimer
/getitimer
system calls, in other words, high
resolution time and timers. See the EXAMPLES section below and the
test scripts for usage; see your system documentation for the
description of the underlying nanosleep
or usleep
, ualarm
,
gettimeofday
, and setitimer
/getitimer
calls.

If your system lacks gettimeofday()
or an emulation of it you don't
get gettimeofday()
or the one-argument form of tv_interval()
.
If your system lacks all of nanosleep()
, usleep()
,
select(), and poll
, you don't get Time::HiRes::usleep()
,
Time::HiRes::nanosleep()
, or Time::HiRes::sleep()
.
If your system lacks both ualarm()
and setitimer()
you don't get
Time::HiRes::ualarm()
or Time::HiRes::alarm()
.

If you try to import an unimplemented function in the use statement
it will fail at compile time.

If your subsecond sleeping is implemented with nanosleep()
instead
of usleep()
, you can mix subsecond sleeping with signals since
nanosleep()
does not use signals. This, however, is not portable,
and you should first check for the truth value of
&Time::HiRes::d_nanosleep
to see whether you have nanosleep, and
then carefully read your nanosleep()
C API documentation for any
peculiarities.

If you are using nanosleep
for something else than mixing sleeping
with signals, give some thought to whether Perl is the tool you should
be using for work requiring nanosecond accuracies.

Remember that unless you are working on a hard realtime system,
any clocks and timers will be imprecise, especially so if you are working
in a pre-emptive multiuser system. Understand the difference between
wallclock time and process time (in UNIX-like systems the sum of
user and system times). Any attempt to sleep for X seconds will
most probably end up sleeping more than that, but don't be surpised
if you end up sleeping slightly less.

The following functions can be imported from this module.
No functions are exported by default.

gettimeofday ()

In array context returns a two-element array with the seconds and
microseconds since the epoch. In scalar context returns floating
seconds like Time::HiRes::time()
(see below).

usleep ( $useconds )

Sleeps for the number of microseconds (millionths of a second)
specified. Returns the number of microseconds actually slept.
Can sleep for more than one second, unlike the usleep
system call.
Can also sleep for zero seconds, which often works like a thread yield.
See also Time::HiRes::usleep()
, Time::HiRes::sleep()
, and
Time::HiRes::clock_nanosleep()
.

Do not expect usleep() to be exact down to one microsecond.

nanosleep ( $nanoseconds )

Sleeps for the number of nanoseconds (1e9ths of a second) specified.
Returns the number of nanoseconds actually slept (accurate only to
microseconds, the nearest thousand of them). Can sleep for more than
one second. Can also sleep for zero seconds, which often works like
a thread yield. See also Time::HiRes::sleep()
,
Time::HiRes::usleep()
, and Time::HiRes::clock_nanosleep()
.

Do not expect nanosleep() to be exact down to one nanosecond.
Getting even accuracy of one thousand nanoseconds is good.

ualarm ( $useconds [, $interval_useconds ] )

Issues a ualarm
call; the $interval_useconds
is optional and
will be zero if unspecified, resulting in alarm-like behaviour.

ualarm(0) will cancel an outstanding ualarm().

Note that the interaction between alarms and sleeps is unspecified.

tv_interval

tv_interval ( $ref_to_gettimeofday [, $ref_to_later_gettimeofday] )

Returns the floating seconds between the two times, which should have
been returned by gettimeofday()
. If the second argument is omitted,
then the current time is used.

time ()

Returns a floating seconds since the epoch. This function can be
imported, resulting in a nice drop-in replacement for the time
provided with core Perl; see the EXAMPLES below.

NOTE 1: This higher resolution timer can return values either less
or more than the core time(), depending on whether your platform
rounds the higher resolution timer values up, down, or to the nearest second
to get the core time(), but naturally the difference should be never
more than half a second. See also clock_getres, if available
in your system.

NOTE 2: Since Sunday, September 9th, 2001 at 01:46:40 AM GMT, when
the time() seconds since epoch rolled over to 1_000_000_000, the
default floating point format of Perl and the seconds since epoch have
conspired to produce an apparent bug: if you print the value of
Time::HiRes::time()
you seem to be getting only five decimals, not
six as promised (microseconds). Not to worry, the microseconds are
there (assuming your platform supports such granularity in the first
place). What is going on is that the default floating point format of
Perl only outputs 15 digits. In this case that means ten digits
before the decimal separator and five after. To see the microseconds
you can use either printf/sprintf with "%.6f"
, or the
gettimeofday()
function in list context, which will give you the
seconds and microseconds as two separate values.

sleep ( $floating_seconds )

Sleeps for the specified amount of seconds. Returns the number of
seconds actually slept (a floating point value). This function can
be imported, resulting in a nice drop-in replacement for the sleep
provided with perl, see the EXAMPLES below.

Note that the interaction between alarms and sleeps is unspecified.

alarm ( $floating_seconds [, $interval_floating_seconds ] )

The SIGALRM
signal is sent after the specified number of seconds.
Implemented using ualarm()
. The $interval_floating_seconds
argument
is optional and will be zero if unspecified, resulting in alarm()-like
behaviour. This function can be imported, resulting in a nice drop-in
replacement for the alarm provided with perl, see the EXAMPLES below.

NOTE 1: With some combinations of operating systems and Perl
releases SIGALRM
restarts select(), instead of interrupting it.
This means that an alarm() followed by a select() may together
take the sum of the times specified for the the alarm() and the
select(), not just the time of the alarm().

Start up an interval timer: after a certain time, a signal ($which) arrives,
and more signals may keep arriving at certain intervals. To disable
an "itimer", use $floating_seconds
of zero. If the
$interval_floating_seconds
is set to zero (or unspecified), the
timer is disabled after the next delivered signal.

Use of interval timers may interfere with alarm(), sleep(),
and usleep()
. In standard-speak the "interaction is unspecified",
which means that anything may happen: it may work, it may not.

In scalar context, the remaining time in the timer is returned.

In list context, both the remaining time and the interval are returned.

There are usually three or four interval timers (signals) available: the
$which
can be ITIMER_REAL
, ITIMER_VIRTUAL
, ITIMER_PROF
, or
ITIMER_REALPROF
. Note that which ones are available depends: true
UNIX platforms usually have the first three, but (for example) Win32
and Cygwin have only ITIMER_REAL
, and only Solaris seems to have
ITIMER_REALPROF
(which is used to profile multithreaded programs).

ITIMER_REAL
results in alarm()-like behaviour. Time is counted in
real time; that is, wallclock time. SIGALRM
is delivered when
the timer expires.

ITIMER_VIRTUAL
counts time in (process) virtual time; that is,
only when the process is running. In multiprocessor/user/CPU systems
this may be more or less than real or wallclock time. (This time is
also known as the user time.) SIGVTALRM
is delivered when the
timer expires.

ITIMER_PROF
counts time when either the process virtual time or when
the operating system is running on behalf of the process (such as I/O).
(This time is also known as the system time.) (The sum of user
time and system time is known as the CPU time.) SIGPROF
is
delivered when the timer expires. SIGPROF
can interrupt system calls.

The semantics of interval timers for multithreaded programs are
system-specific, and some systems may support additional interval
timers. For example, it is unspecified which thread gets the signals.
See your setitimer()
documentation.

getitimer ( $which )

Return the remaining time in the interval timer specified by $which
.

In scalar context, the remaining time is returned.

In list context, both the remaining time and the interval are returned.
The interval is always what you put in using setitimer()
.

clock_gettime ( $which )

Return as seconds the current value of the POSIX high resolution timer
specified by $which
. All implementations that support POSIX high
resolution timers are supposed to support at least the $which
value
of CLOCK_REALTIME
, which is supposed to return results close to the
results of gettimeofday
, or the number of seconds since 00:00:00:00
January 1, 1970 Greenwich Mean Time (GMT). Do not assume that
CLOCK_REALTIME is zero, it might be one, or something else.
Another potentially useful (but not available everywhere) value is
CLOCK_MONOTONIC
, which guarantees a monotonically increasing time
value (unlike time(), which can be adjusted). See your system
documentation for other possibly supported values.

clock_getres ( $which )

Return as seconds the resolution of the POSIX high resolution timer
specified by $which
. All implementations that support POSIX high
resolution timers are supposed to support at least the $which
value
of CLOCK_REALTIME
, see clock_gettime.

clock_nanosleep ( $which, $nanoseconds, $flags = 0)

Sleeps for the number of nanoseconds (1e9ths of a second) specified.
Returns the number of nanoseconds actually slept. The $which is the
"clock id", as with clock_gettime() and clock_getres(). The flags
default to zero but TIMER_ABSTIME
can specified (must be exported
explicitly) which means that $nanoseconds
is not a time interval
(as is the default) but instead an absolute time. Can sleep for more
than one second. Can also sleep for zero seconds, which often works
like a thread yield. See also Time::HiRes::sleep()
,
Time::HiRes::usleep()
, and Time::HiRes::nanosleep()
.

Do not expect clock_nanosleep() to be exact down to one nanosecond.
Getting even accuracy of one thousand nanoseconds is good.

clock()

Return as seconds the process time (user + system time) spent by
the process since the first call to clock() (the definition is not
"since the start of the process", though if you are lucky these times
may be quite close to each other, depending on the system). What this
means is that you probably need to store the result of your first call
to clock(), and subtract that value from the following results of clock().

The time returned also includes the process times of the terminated
child processes for which wait() has been executed. This value is
somewhat like the second value returned by the times() of core Perl,
but not necessarily identical. Note that due to backward
compatibility limitations the returned value may wrap around at about
2147 seconds or at about 36 minutes.

stat

stat FH

stat EXPR

As stat but with the access/modify/change file timestamps
in subsecond resolution, if the operating system and the filesystem
both support such timestamps. To override the standard stat():

Test for the value of &Time::HiRes::d_hires_stat to find out whether
the operating system supports subsecond file timestamps: a value
larger than zero means yes. There are unfortunately no easy
ways to find out whether the filesystem supports such timestamps.
UNIX filesystems often do; NTFS does; FAT doesn't (FAT timestamp
granularity is two seconds).

A zero return value of &Time::HiRes::d_hires_stat means that
Time::HiRes::stat is a no-op passthrough for CORE::stat(),
and therefore the timestamps will stay integers. The same
thing will happen if the filesystem does not do subsecond timestamps,
even if the &Time::HiRes::d_hires_stat is non-zero.

In any case do not expect nanosecond resolution, or even a microsecond
resolution. Also note that the modify/access timestamps might have
different resolutions, and that they need not be synchronized, e.g.
if the operations are

C API

In addition to the perl API described above, a C API is available for
extension writers. The following C functions are available in the
modglobal hash:

name C prototype

--------------- ----------------------

Time::NVtime double (*)()

Time::U2time void (*)(pTHX_ UV ret[2])

Both functions return equivalent information (like gettimeofday
)
but with different representations. The names NVtime
and U2time
were selected mainly because they are operating system independent.
(gettimeofday
is Unix-centric, though some platforms like Win32 and
VMS have emulations for it.)

Here is an example of using NVtime
from C:

double (*myNVtime)(); /* Returns -1 on failure. */

SV **svp = hv_fetch(PL_modglobal, "Time::NVtime", 12, 0);

if (!svp) croak("Time::HiRes is required");

if (!SvIOK(*svp)) croak("Time::NVtime isn't a function pointer");

myNVtime = INT2PTR(double(*)(), SvIV(*svp));

printf("The current time is: %f\n", (*myNVtime)());

DIAGNOSTICS

useconds or interval more than ...

In ualarm() you tried to use number of microseconds or interval (also
in microseconds) more than 1_000_000 and setitimer() is not available
in your system to emulate that case.

negative time not invented yet

You tried to use a negative time argument.

internal error: useconds < 0 (unsigned ... signed ...)

Something went horribly wrong-- the number of microseconds that cannot
become negative just became negative. Maybe your compiler is broken?

CAVEATS

Notice that the core time() maybe rounding rather than truncating.
What this means is that the core time() may be reporting the time
as one second later than gettimeofday()
and Time::HiRes::time()
.

Adjusting the system clock (either manually or by services like ntp)
may cause problems, especially for long running programs that assume
a monotonously increasing time (note that all platforms do not adjust
time as gracefully as UNIX ntp does). For example in Win32 (and derived
platforms like Cygwin and MinGW) the Time::HiRes::time() may temporarily
drift off from the system clock (and the original time()) by up to 0.5
seconds. Time::HiRes will notice this eventually and recalibrate.
Note that since Time::HiRes 1.77 the clock_gettime(CLOCK_MONOTONIC)
might help in this (in case your system supports CLOCK_MONOTONIC).